H.E.S.S.

High Energy Stereoscopic System

News about HESS J1858+020

January 2010

The very high energy gamma ray source HESS J1858+020
is located about 0.6 degr. off the Galactic plane, which is indicated by the
dashed line. The color indicates the strength of the gamma-ray emission. The
green circle illustrates the point spread function of the instrument.

HESS J1858+020 is among the sources discovered by H.E.S.S.
in its survey of the Galactic plane. Unlike most other sources, HESS J1858+020
represents a rather compact emission region of about 5' in size. Is features a
relatively hard gamma-ray spectrum, extending beyond 10 TeV. Lacking any obvious
counterpart, the source was classified as "unidentified" at the time of the
discovery.

New clues regarding the nature of HESS J1858+020 come from
two recent publications:
Green (2009)
reports on the re-classification of the extended radio source G35.6-0.4 as a
supernova remnant, based on VLA radio data at 1.4 GHz which resolve the source,
based on the non-thermal spectrum of the radio emission, and based on the lack of infrared
emission. G35.6-0.4 (Fig. 1,
Fig. 2) extends from the location of HESS J1858+020
towards the Galactic plane.
Green (2009)
estimates the age of the remnant to about 30000 years, and its distance to about
10 kpc, both numbers with large uncertainty.
Paron and Giacani
(2009) report the presence of a molecular cloud of about 5000 solar masses
at edge of the remnant G35.6-0.4, coincident with the H.E.S.S. gamma-ray source
(Fig. 2, Fig. 3).
The cloud is visible in 13CO radio data, selecting a velocity range 51 to 59
km/s, corresponding to the estimated distance to G35.6-0.4.

The implied scenario is then a supernova remnant which
illuminates a nearby molecular cloud where gamma rays are produced by
interacting protons accelerated in the remnant (e.g.
Gabici et al. 2009).
The emission from the remnant itself is too faint to be detectable, but the 5000
solar mass cloud boosts proton interaction rates. If this interpretation is
correct, the system would demonstrate proton acceleration by supernova remnants.
However, an alternative explanation could be the formation of massive stars
inside the cloud, which in their various stages of evolution could also
accelerate particles. Analyzing Spitzer infrared images,
Paron and Giacani
(2009) indeed find evidence for star formation inside the cloud (Fig. 3).

Fig. 1:
VGPS 1.4 GHz radio image of G35.6-0.4 (the lower extended object; the bright
region near the top is the thermal source G35.5-0.0). Note that this and the
following images are in Galactic coordinates, whereas the top image is in
RA-Dec coordinates. From
Green (2009).

Fig. 2:
Three-color radio / infrared image of the region of G35.6-0.4 (red = 8 mu IR,
green = 24 mu IR, blue = 20 cm radio emission). The black contours delineate
radio emission from the remnant G35.6-0.4. The white contours show the
density of molecular gas, traced by its 13CO radio emission. The yellow
circle indicates the location and approximate size of HESS J1858+020. From
Paron and
Giacani (2009).

Fig. 3:
Zoomed-in three-color infrared image (8 mu, 4.5 mu, and 3.6 mu) of the vicinity of HESS J1858+020;
the gamma-ray source is indicated by the yellow circle. The white
contours again show the density of molecular gas, traced by its 13CO radio
emission. The crosses show star-formation candidates, identified by their
spectra. From
Paron and
Giacani (2009).